Update avro_util.erl

[AlfUA]

To parse a schema that contains the following structure (an object as a value):
{
"name":Limit",
"type":["null",{
"type":"bytes",
"scale":2,
"precision":64,
"connect.version":1,
"connect.parameters":{"scale":"2"},
"connect.name":"org.apache.kafka.connect.data.Decimal",
"logicalType":"decimal"
}],
"default":null
}.
So we have to add an extra function clause that should handle the nested tuple, otherwise, schema parsing fails.

[coveralls]

Pull Request Test Coverage Report for Build 559

• 3 of 4 (75.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.06%) to 99.941%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/avro_util.erl	3	4	75.0%

Totals
Change from base Build 557:	-0.06%
Covered Lines:	1697
Relevant Lines:	1698

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💛 - Coveralls